<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text"><button>按钮</button>
    <div></div>
    <script>
        var input=document.querySelector('input');
        var button=document.querySelector('button');
        var div=document.querySelector('div');
        button.onclick=function(){
            var str=input.value.trim();
            if(str===''){
                alert('请输入内容');
            }else{
                console.log(str);
                console.log(str.length);
                div.innerHTML=str;

            }
        }
    </script>
</body>
</html>